1. Field of the Invention
The present invention relates generally to digital servo systems, and more particularly, to a digital servo system in which controlling of a rotational speed of a rotary body such as a cylinder motor and a capstan motor of a video tape recorder (referred to as VTR hereinafter) is achieved using a microcomputer.
2. Description of the Background Art
Conventionally, in a VTR, there have been provided a cylinder motor for driving the rotation of a rotary head and a capstan motor for driving the travelling of a tape. During the operation of the VTR, rotational speeds of the above described cylinder motor and capstan motor used as driving means are digitally servo controlled in order to accurately control a rotational speed of the rotary head and a traveling speed of the tape.
As described in Japanese Patent Laying-Open No. 61-271644, a digital servo control of a rotational speed of such a motor as a cylinder motor and a capstan motor is performed based on a signal indicating an actually detected rotational speed of the motor, that is, a FG (Frequency Generator) signal comprising a predetermined number (for example 24 numbers) of pulses generated per one rotation of the motor. The higher a frequency of the FG signal, that is, a sampling frequency for a digital servo control is the better will a response speed of a servo loop and response characteristics be of the servo loop for a load fluctuation, that is, controllability over disturbance. This is because if the sampling frequency is low, it becomes difficult to perform a servo control quickly in response to a disturbance which may be caused.
In order to increase the sampling frequency for the servo control, that is, a frequency of the FG signal, it, e.g., has been proposed to improve a structure of a detector itself of the FG signal. More specifically, by increasing the number of magnets constituting the FG signal detector, the number of FG pulses generated per one rotation of the motor is increased to increase the frequency of the FG signal.
However, the increased number of the magnets of the FG signal detector results in a large-sized FG signal detector, which leads to a large-sized motor. Therefore, such a method is not practical to be adopted.